Heater switching cathode for magnetrons



June 29, 1965 D. M. JEWART Y, I 3,192,436

HEATER SWITCHING CATHODE FOR MAGNETRONS Filed June 2. 1961 INVENTORDona/a M. Jen arr BY ATTORNEY United States Patent 3,192,436 HEATERSWITCHING CATHODE FOR MAGNETRONS Donald M. Jewart, Williamsport, Pa.,assignor, by mesne assignments, to Litton Industries, Inc., BeverlyHills,

Califi, a corporation of Delaware Filed June 2, 1961, Ser. No. 114,350 4Claims. (Cl. 315-73) This invention relates to electron dischargedevices employing an indirectly heated cathode and an anode and moreparticularly to the control of the current through the cathode heater.

In many electron discharge devices employing an indirectly heatedcathode and particularly in magnetrons, it is necessary to control thetemperature of the cathode. During operation of these devices many ofthe electrons follow a cardioid path and back-bombard the cathode,thereby causing undesirable overheating. The life characteristics of thecathode are deleteriously affected by the occurrence of overheating.

In order to reduce the harmful effects of high operating cathodetemperatures, efforts have been made to control the cathode heatercurrent. For this purpose, circuitry external to the tube, such asmanual potentiometer control or relay control, has been employed toregulate the cathode heater current. A control system of this type'isexpensive and diificult to operate and monitor.

Accordingly, an object of this invention is to decrease the undesirableoverheating of the cathode by auto1nati cally temporarily breaking theheater circuit of the device to regulate the heating.

Another object is to'provide an internal means for controlling heatercurrent, thereby alleviating the need for external control circuitry.

A further object of the invention is to permit a wider range oftolerance on heater power during warm up by providing automatic cathodetemperature control thereafter.

The foregoing objects are achieved in one aspect of the invention by theprovision of an indirectly heated cathode structure comprising twocathode assembly structures'of dissimilar metals having differentthermal coefficients of expansion. These two structures are suitablyfastened together at one end. The opposite end of each structurecontains an electrical contact which is part of the heater circuit. Innormal operation, the contacts on the ends of the structures are closedto allow current to flow in the heater circuit. When the cathode reachesa prescribed critical temperature, the longitudinal differential thermalexpansion of the two structures causes the contacts on the ends of thesleeves to temporarily sepa rate, thereby breaking the heater circuit.Thus the cathode temperature is allowed to reduce to a safe operationalrange whereat, by thermal longitudinal contraction, the contacts areagain made to reactivate the heater circuit. In this manner, the cathodeoperational temperature is controlled within thermal limits compatiblewith desired tube operation.

For a better understanding of the invention, reference is made to thefollowing description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 portrays a fragmentary sectional view of a magnetron tube andshows the portions in the interaction space normally employed in thesedevices; 7

FIG. 2 shows a modification of a switch structure shown in FIG. 1;

FIG. 3 is a perspective view of a leaf spring employed in the modifiedform of invention shown in FIG. 2.

In FIGURE 1 the numerals 10 and 12 designate pole pieces held in thebody 14 of a magnetron type tube.

The magnetron is provided with ananode 16 held between the pole pieces10 and 12. The cathode, indicated generally as 18, passes axiallythrough the pole pieces 10 and anode 16, and into a space in the polepiece 12 wherein it is free to expand longitudinally of its axis. Thecathode sleeve is a composite structure comprised of an innercylindrical sleeve 20 of a material such as molybdenum having arelatively low thermal co-eflicient of expansion nested in an outersleeve 22 such as nickel having a relatively high thermal co-efficientof expansion. The outer sleeve is coated opposite the anode with anelectron emissive coating 24 such as the triple alkaline earthcarbonates of barium, strontium, and calcium. Circular flanges 26 may beprovided about the outer sleeve to prevent stray emission from thecathode from reaching the pole pieces. The inner and outer sleeves areimmovably brazed with copper or otherwise suitably joined together asindicated at 28 but are otherwise unattached. However, the inner sleeve20 lies in close contact for excellent heat transfer and longitudinalexpansion relationships with the outer sleeve 22. The cathode issupported near the end jointure of the sleeves by mechanically andelectrically joining the outer sleeve 22 to a metallic cone 3%, which issuitably insulatively mounted on a fixed part of the magnetron, as onthe pole piece 10. At the free end of the sleeve 20 is fixedly mounted acentrally perforated ceramic button 32 surmounted by a noncorrosivecontact plate 34. A cooperating non-corrosive contact button 36 is fixedonto the free end of the outer sleeve 22. Firmly attached to the contactplate 34 is the end 37 of the cathode heater 38. A retainer ring 42biting into the wire keeps the contact plate 34 firmly against theceramic button 32. The opposite end 44 of heater 38 leads to a suitablecur-rent supply (not shown), the other side of the supply beingconnected to end lead 46 which is in turn electrically connected to themetallic cone 30, sleeve 22, and button 36.

When the cathode is unheated, the outer sleeve 22 is contracted inlength relative to the inner sleeve 20 and the metallic button 36 is incontact with the plate 34. When the electric circuit to the magnetron isclosed externally of the tube, the heater circuit is closed and thecathode will become active and electron emission will take place. Shouldthe cathode 18 become too hot for some reason such as fromback-bombardment of the cathode with electrons, the outer sleeve 22 willheat and elongate to a greater degree than the inner sleeve due to itsgreater co-efficient of thermal expansion and thereby open the circuitat the free ends of the sleeves by moving button 36 away from plate 34.The heater circuit remains open until the cathode cools sufficiently toallow sleeve 22 to retract and carry button 36 into contact with plate34 again.

Thus, by repeated operation of the cathode sleeve contacts 34 and 36, asubstantially steady state cathode temperature will be maintained.Preferably, there is sufficient yield in the metals of the sleeves 20and 22 to allow for the small amount of contraction when the cathodestructure is cold and the contacts are together. However, where there isa wide variation in temperature between a cold non-operating tube andits subsequent operational temperature, additional yield of the partsmay be desired to prevent deforming or rupturing of the parts or joints.Therefore, additional yield of the parts may be effected by means suchas is shown in FIG. 2. In this form of the invention, the outer sleevedesignated as 22' in FIG. 2 is slightly longer than the outer sleeve 22in FIG. 1 to allow room for inclusion of a compression member 48.Contact 36 is replaced by 36 which is of sufficient thickness to beslidable within sleeve 22 for movement toward and from contact plate 34.The substantially N- aroaeee shaped leaf spring 48 has substantiallynormal parallel end legs brazed to a face of the contact 36' and to theinner face of a cover plate 5% which in turn is brazed to the free endof outer cathode sleeve 22'. This spring absorbs some of the contractionstresses which may otherwise occur if sleeve 22 contracts beyond thelimit where contacts 34 and 36' are pressed together.

The present invention provides an efiicient means for automaticallycontrolling heater current and resultant cathode temperature in anelectron discharge device by providing an internal make-and-break heatercircuit switching mechanism.

Although several embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thescope of the invention as defined by the appended claims.

Having thus described the invention, what is claimed is:

1. A thermionic indirectly heated cathode structure comprising an innerand an outer sleeve in nested relationship and fastened together nearadjacent support ends of the sleeves, said sleeves being of metalshaving different co-eificients of heat expansion, an electrical contactcarried by each of the sleeves substantially at their free ends movablebetween an engaging and non-engaging position relative to one another, acathode heater within the inner sleeve having one end connected to oneof the contacts and the other end extending away from the cathode, andan electrical connection to the second one of the contacts, whereby theheater current may be controlled by the relative thermal expansion ofthe telescoped sleeves in accordance with the relative positions of saidcontacts.

2. A thermionic indirectly heated cathode structure comprising an innersleeve having a relatively low thermal co-eflicient of expansion, anouter sleeve having a greater thermal co-efiicient of expansion with thetwo sleeves being mounted in nesting relationship and contacting eachother to promote heat transfer and enable longitudinal diilerentialexpansion, said sleeves being fastened together at one end and eachprovided with a contact at their free ends for movement between anengaging and non-engaging position, electron emissive material on theouter sleeve, a cathode heater positioned within said inner sleevehaving one terminal connected to the inner sleeve contact, and anelectrical connection leading to the contact carried by the outersleeve, whereby the heater current may be controlled by the relativethermal expansion of the sleeves in accordance with the relative posi-,tions or" said contacts.

3. A thermionic indirectly heated cathode structure comprising an innersleeve having a relatively low thermal co-ellicient of expansion, anouter sleeve of greater thermal co-efiicient of expansion with the twosleeves being mounted in nesting relationship and contacting each otherto promote heat transfer and enable longitudinal differential expansion,said sleeves being fastened together at one end and each provided with acontact at their free ends for movement between an engaging andnonengaging position, said outer sleeve contact being resilientlymounted thereon, electron emissive material on the outer sleeve, acathode heater positioned within said inner sleeve iaving one terminalconnected to the inner sleeve contact, and an electrical connectionleading to the contact carried by the outer sleeve, whereby the heatercurrent may be controlled by the relative thermal expansion of thesleeves in accordance with the relative positions of said contacts.

4. A magnetron having a pair of pole pieces and an anode supportedtherebetween, a cathode centrally located in the magnetron andtraversing the anode, said cathode containing emissive material on aportion thereof opposite the anode and a support for the cathode at oneend thereof only, saidcathode having an outer sleeve and an inner sleevein nested relationship attached to one another at one end thereof, saidsleeves being mounted to provide heat transfer and dillerentiallongitudinal expansion, the outer sleeve being of a metal having ahigher thermal co-efficient of expansion than the inner sleeve, acontact carried by each of the sleeves substantially at the free endsthereof with the inner sleeve contact being insulated therefrom, acathode heater Within the inner sleeve having a lead-in connected to oneend of the heater, the other end of the heater terminating in thecontact carried by the inner sleeve, second lead-in for the heaterhaving electrical connection with the contact on the outer sleeve,whereby the heater current may be controlled by the relative thermalexpansion of the sleeves.

References Cited by the Examiner UNITED STATES PATENTS 1,772,002 8/30Harper. 11,969,105 8/34 Smulski ZOO-422.3 2,286,929 6/42 Pond. 2,711,4576/55 Wise. 12,957,100 10/60 Espersen et al 313-337 X GEORGE N. WESTBY,Primary Examiner. RALPH G. NILSON, Examiner.

1. A THERMIONIC INDIRECTLY HEATED CATHODE STRUCTURE COMPRISING AN INNERAND AN OUTER SLEEVE IN NESTED RELATIONSHIP AND FASTENED TOGETHER NEARADJACENT SUPPORT ENDS OF THE SLEEVES, SAID SLEEVES BEING OF METALSHAVING DIFFERENT CO-EFFICIENTS OF HEAT EXPANSION, AN ELECTRICAL CONTACTCARRIED BY EACH OF THE SLEEVES SUBSTANTIALLY AT THEIR FREE ENDS MOVABLEBETWEEN AN ENGAGING AND NON-ENGAGING POSITION RELATIVE TO ONE ANOTHER, ACATHODE HEATER WITHIN THE INNER SLEEVE HAVING ONE END CONNECTED TO ONEOF THE CONTACTS AND THE OTHER END EXTENDING AWAY FROM THE CATHODE, ANDAN ELECTRICAL CONNECTION TO THE SECOND ONE OF THE CONTACTS, WHEREBY THEHEATER CURRENT MAY BE CONTROLLED BY THE RELATIVE THEREMAL EXPANSION OFTHE TELESCOPED SLEEVES IN ACCORDANCE WITH THE RELATIVE POSITIONS OF SAIDCONTACTS.